(A) Mps1 phosphorylates (P) three different proteins to promote the assembly of the mitotic checkpoint complex. It phosphorylates the kinetochore protein KNL1 to recruit the checkpoint protein complex Bub1-Bub3 to KNL1 (1). It phosphorylates Bub1, which allows this protein to interact with another checkpoint protein, Mad1 (2). It also phosphorylates Mad1, which promotes the binding of Mad2 to the regulatory protein Cdc20 (3). Ji et al. propose that phosphorylated Mad1 binds to Cdc20, thereby positioning the latter for capture by Mad2. (B) The checkpoint (represented by the STOP sign) is only active when Mps1 has phosphorylated all three proteins, KNL1, Bub1, and Mad1. (C) Checkpoint activity (y-axis) plotted as a function of Mps1 kinase activity (x-axis) for the phosphorylation of one (P), two (PP) or all three sites (PPP).